Silicon wafers by epitaxial deposition

ABSTRACT

A system for depositing thin single crystal silicon wafers by epitaxial deposition in a silicon precursor depletion mode with cross-flow deposition may include: a substrate carrier with low total heat capacity, high emissivity and small volume; a lamp module with rapid heat-up, efficient heat production, and spatial control over heating; and a manifold designed for cross-flow processing. Furthermore, the substrate carrier may include heat reflectors to control heat loss from the edges of the carrier and/or heat chokes to thermally isolate the carrier from the manifolds, allowing independent temperature control of the manifolds. The carrier and substrates may be configured for deposition on both sides of the substrates—the substrates having release layers on both sides and the carriers being configured to have equal process gas flow over both surfaces of the substrate. High volume may be addressed by a deposition system comprising multiple mini-batch reactors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/491,152 filed May 27, 2011, incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates generally to fabrication of siliconwafers, and more particularly to fabrication of silicon wafers byepitaxial deposition.

BACKGROUND

Crystalline silicon provides high solar cell efficiencies, η, of up toapproximately 23% in commercial production environments, together withadvantages, when compared with thin film solar cells such as CIGS, CdTe,etc., of (a) availability, (b) environmentally friendliness, and (c)demonstrated long lifetimes and associated technology maturity. However,crystalline silicon traditionally has a higher photovoltaic module costthan competitive thin-film panels. (A module is the unit that actuallygenerates power and includes a frame holding a number of solar cells,the latter being electrically connected together in series and then toan inverter.) A large part of this cost comes from the cost ofmanufacturing the silicon wafers (currently ˜180 μm thick), whichincludes the cost of polysilicon production, ingot formation, andwafering (wire sawing the ingot and finishing the cut wafers).

Thin film processes (amorphous silicon, CIGS and CdTe) have in recentyears evoked excitement because of potentially lower costs thancrystalline silicon due to less consumables and large-format, integratedprocessing. However, in general thin film photovoltaic (PV) modulestypically have efficiencies substantially less than that of crystallinesilicon modules. A typical mono-crystalline module has an efficiency of15-16% (with some modules as high as 20%), whereas the best case forthin film modules is currently 11%. In addition, the cost advantages ofmost thin film processes have not been conclusively demonstrated. As aresult, crystalline silicon (both monocrystalline and multicrystalline)commands more than an 80% share of the current PV market, atapproximately 14 GW for 2010. (Quantities of PV modules are commonlymeasured by their total power output in Watts.)

A total installed cost for PV modules of less than $2.50/Wp (Wp is Wattpeak, referring to the maximum achievable power) is attractive todaysince in many parts of the world it represents grid-parity with modestincentives. (Grid parity refers to the cost per Watt from a PV modulebeing the same as the cost per Watt available over an electricitydistribution grid, where a typical grid I fed by a multitude of powersources such as coal, oil and gas power stations.

Manufacturers have been particularly effective at reducing themanufacturing costs of conventional crystalline silicon (bothmonocrystalline and multicrystalline) technologies—silicon PV moduleshave come down in price from more than $4 per Wp in 2006 to roughly$1.80 per Wp in 2010. The most cost efficient integrated manufacturersof PV wafers, cells and modules currently have costs of goods sold (costof making PV module, including material, labor and overheads) for a PVmodule approaching $1.10 per Wp. These cost reductions over the pastfour years have in part been due to the following technicalimprovements.

First, silicon usage has been reduced from 10 gms per Watt to roughly6.5 gms per Watt due to a reduction in the solar cell thicknesses andimproved wafer sawing processes with lower kerf losses.

Second, cell efficiencies have been improved by fine line printing,improved front-side reflection and passivation control, higher quality(higher lifetime) materials, etc. The average silicon cell efficiencyhas increased from approximately 14% to 17%, with a number ofmanufacturers reporting 18% cell efficiencies for their monocrystallinesilicon PV cells.

Third, manufacturing has been more fully integrated—there has been atrend to concatenate the various parts of the silicon PV modulemanufacturing process (polysilicon, wafer, cell, and module) for greatercost-effectiveness, and there is now world wide access to systemintegrators and installers. For example, in 2010 integratedmanufacturing resulted in a cost structure that gives a COGS for PVmodules of approximately $1.10.

It is clear that, for the foreseeable future, single crystal silicon PVmodules can continue to be a leader in PV and will compete effectivelywith thin film PV technologies as grid parity is reached, providingsingle crystal silicon PV module manufacturing can continue with furthercost reductions. However, the source of further cost reductions is notimmediately apparent.

PV cell and module costs are approaching asymptotic levels with thedeployment of large scale production equipment and single crystalsilicon cell efficiencies near 19% have been reached, and furtherincreases in cell efficiency may only be achieved at increased cost.

However, further cost reductions may come from a significant reductionin silicon and wafering costs since they now represent the largestportion of the module cost structure. Silicon and wafering costs wouldbe reduced by (a) further reductions in polysilicon production costs,(b) improvements in the crystal growth processes and/or (c) sawingwafers thinner with high yield and low kerf loss. Similarly, acontinuous Czochralski crystal growth process may provide incrementalcost improvements, but radical cost reductions are unlikely. However,sawing wafers to less than 180 μm has been plagued by lower yields anddisproportionately higher kerf losses, as the wire saw technology startsto reach fundamental mechanical limits. Hence, reduction of siliconusage requires looking at new technologies that can bypass thepolysilicon, ingoting, and wafering steps altogether.

A further incentive for bypassing these steps is that polysilicon iscurrently the step that gates the expansion of silicon PV moduleproduction capacity, primarily due to the capital cost associated withthe polysilicon step alone. In fact, the capital costs—the one-timeset-up costs for manufacturing—for polysilicon, ingoting and waferingdominate the total capital costs for production of silicon PV devices.Not only do polysilicon, ingoting and wafering have the highest capitalcost (accounting for approximately $2 per Wp out of a total of $3 per Wpfor setting up for production of solar cell modules), they also requirethe most infrastructure in terms of land, gases, water, etc.

It is evident from the above discussion that the crystalline silicon PVindustry has made great strides in reducing cost and remainingcompetitive with thin film PV. However, to get to unsubsidized gridparity at roughly $0.08 per kWh (which is equivalent to approximately $2per Wp), the total COGS needs to be reduced to approximately $0.80 perWp for crystalline silicon PV wafers, which is what is required to getto system installed costs for crystalline silicon PV modules of $2 perWp. This reduction is difficult for conventional silicon PV technology—atechnology that has already benefitted from the cost reductions due tomass production and innovations that originate in the conventionalsilicon semiconductor industry. Hence, it is evident that there is aneed for cheaper new processes that can replace the current costlyprocesses associated with polysilicon, ingoting and wafering, along withan attendant decrease in capital costs.

Epitaxial deposition of crystalline silicon substrates for PV moduleproduction allows a drastic simplification of the supplychain—eliminating the need for polysilicon, ingoting and wafering.However, the challenge is utilizing an epitaxial deposition process at acost which is comparable if not lower than traditional processes formanufacturing the single crystal silicon wafers.

Epitaxial deposition of thin films of silicon is a common process stepin the fabrication of semiconductor devices such as integrated circuits.To enable a high yield for the integrated circuits, this epitaxialdeposition step requires that the deposited silicon is of very highquality with a very stringent thickness uniformity. This can only beachieved at lower deposition rates and therefore most of thesemiconductor epitaxial reactors are optimized for highly uniform, lowdefect deposition rates of 0.1 to 1 μm per min. Today almost alladvanced high performance CMOS (complementary metal oxide semiconductor)devices are built on such epitaxial layers.

Most conventional epitaxial batch reactors used in the semiconductorindustry depend on diffusion to supply reactants—such as trichlorosilane(TCS) and hydrogen—to the center of wafers which inherently results in ahigher concentration of TCS on the wafer edge since the gas flow istypically along the periphery of the wafer stack. Thus in order tomaintain within wafer and wafer-to-wafer film thickness uniformity, suchreactors have to be run in a reaction-rate limited regime at lowertemperatures where deposition rates are much lower. Furthermore, fordeposition of thin epitaxial films where film quality and uniformity isvery important and the device value can accommodate the high processcost, high temperature epitaxial growth is utilized, typically in asingle wafer reactor. In this high temperature process chemical vapordeposition (CVD) of TCS is done under a mass transport limited regimewhere the growth is dependent on the mass transport of TCS to thereaction surface across a boundary layer. However, high temperatureprocessing of single wafers is cost prohibitive for all except the veryhigh value devices such as cutting edge processors.

To circumvent limitations of reactors which rely on diffusion ofreactants, some semiconductor epitaxial reactors are designed to have aconstant growth rate across the surface of the wafer by forming a stableboundary layer across which there is a constant availability ofprecursor species. This is typically achieved using the followingtechniques.

First, the wafer is rotated, which ensures that the gas velocity and theboundary layers are constant across the wafer surface. However, havingto rotate the wafers limits reactor types to either single waferreactors or small batch reactors.

Second, a sufficient supply of TCS is provided such that the TCS isavailable in the vicinity of the reactant surface without incurringprecursor depletion. This can be achieved by having multiple precursorgas injection points to flood the deposition chamber with TCS.Consequently, in these reactors the TCS utilization is typically onlyaround 5%.

Third, the temperature of the wafers is typically less than 1050° C.,thus the growth rates are less than 1 μm per min. Although higher growthrates can be achieved in these reactors, design constraints due to therequirement for wafer rotation at deposition temperatures generallykeeps the operating temperature in the 1,000 to 1,050° C. range. Forsemiconductors this low deposition rate is acceptable since thethickness uniformity and epitaxial quality in-terms of low defectdensity are paramount.

Fourth, the substrates must be heated-up to deposition temperature andthen cooled-down after deposition is complete—this typically takes onehour and is on top of the deposition time for throughput calculations ifdone in the deposition chamber. Note that these deposition systemstypically utilize inductive heating systems, rated at roughly 200 kW.

FIG. 1 (from Sandra Bau, High-temperature CVD silicon films forcrystalline silicon thin-film solar cells, Ph.D. dissertation,University of Konstanz 2003 (see page 18, FIG. 3.4), available athttp://kops.ub.uni-konstanz.de/handle/urn:nbn:de:bsz:352-opus-11305)shows a plot of growth rate of silicon from TCS as a function of keyvariables—the substrate temperature and the TCS to H₂ ratio. In the caseof semiconductor applications, for which commercial epitaxial reactorshave been designed, the growth rate must be controlled with a highdegree of accuracy, which requires operation of the process underconditions where fluctuations in temperature and TCS to H₂ ratio resultin no appreciable change in the deposition rate. Thus, the operatingregion for commercial semiconductor epitaxy is on the flat part of thecurve as indicated in FIG. 1.

The following assumptions are used to estimate the cost of epitaxialdeposition of a 180 μm crystalline silicon wafer on a 125 mm substrate,assuming production in a commercial semiconductor epitaxial reactor withan 8 wafer batch capacity. At a 1 μm per minute deposition rate (seeFIG. 1), and including heat-up and cool-down time of 60 minutes, thetotal production time is 240 minutes. The throughput of a single systemoperating with an 8 wafer batch is8×60/240=2 wafers per hour.The depreciation cost per wafer over a 7 year period and assuming a 90%up-time, a 90% utilization and a $1.2 M ASP (application specificproduct) for the reactor is given by$1.2 M/(2×0.9×0.9×24×350×7)=$12.60 per wafer.The TCS consumed per wafer, at 5% utilization is 650 grams per wafer.Assuming the commercial price of TCS is $3 per kg, the cost of TCS perwafer is $2 per wafer. The total power used to produce a batch of 8wafers is estimated to be 200 kW, and with a cost of power per wafer of4¢ per kWh, this gives a cost of power of $1 per wafer. Other gases andconsumables are estimated to be approximately $1 per wafer.Consequently, the total cost of epitaxial deposition is estimated to be$16.6 per wafer.

Using epitaxial deposition for producing crystalline silicon wafers forPV modules is attractive since it would allow a drastic simplificationof the supply chain, although the cost of the epitaxial deposition mustbe comparable if not lower than traditional processes for manufacturingthe silicon wafers. As described above, the cost of silicon wafers forPV applications needs to be close to $0.80 per wafer. However, usingcommercial semiconductor epitaxial reactors it is estimated to costsubstantially more than this—roughly $16.6 per wafer. Clearly, there isa need for an epitaxial deposition reactor that will bring the cost perwafer down by roughly a factor of 25 or more.

SUMMARY OF THE INVENTION

The present invention as described herein is a transformative technologyfor the reduction of manufacturing costs of single crystal siliconphoto-voltaics based on the method involving direct-epitaxial depositionof the silicon wafers. Monocrystalline silicon wafers are produceddirectly from the gas phase, using technology that bypasses the steps of(a) polysilicon production, (b) crystal growth, and (c) wafering of theingots. This enables a substantial reduction in silicon usage andproduction cost with a potential for manufacturing costs approachinggrid parity for high efficiency PV modules. Key innovations in processand equipment technologies, which alone or in combination, enablestringent cost and technology targets, as well as scalability intovolume production, include: epitaxial deposition in a silicon precursordepletion mode with cross-flow deposition to compensate for the decreasein silicon deposition rate on substrates along the direction of gasflow; a deposition system comprising multiple mini-batch reactors; asubstrate carrier with low total heat capacity, high emissivity andsmall volume; a lamp module with rapid heat-up, efficient heatproduction, and spatial control over heating; and a manifold designedfor cross-flow processing. Furthermore, the substrate carrier mayinclude heat reflectors to control heat loss from the edges of thecarrier and/or heat chokes to thermally isolate the carrier from themanifolds, allowing independent temperature control of the manifolds.The carrier and substrates may be configured for deposition on bothsides of the substrates—the substrates having release layers on bothsides and the carriers configured to have equal process gas flow overboth surfaces of the substrate. Furthermore, the deposition system maybe configured to allow removal of substrate carriers from the reactorwhile the substrate carrier is still at 400 to 600° C., thus reducingprocess cycle time through the reactor. Furthermore, the substratecarrier may be configured to engage with the gas manifold in thereaction chamber of the reactor to contain process gases within thesubstrate carrier, thus enabling a low frequency of cleaning for thereaction chamber of the reactor and a high reactor availability.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those ordinarily skilled in the art upon review ofthe following description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1 is a plot of deposition rate of epitaxial silicon as a functionof Cl/H ratio in the deposition gas calculated for a horizontalatmospheric pressure epitaxial reactor;

FIG. 2 is a process flow for epitaxial silicon substrate growth,according to embodiments of the present invention;

FIG. 3 is a perspective view of a system of mini-batch epitaxialreactors configured for high throughput processing, according toembodiments of the present invention;

FIG. 4 is a perspective view of an epitaxial reactor, according toembodiments of the present invention;

FIG. 5A is a partial cutaway view along X-X of the epitaxial reactor ofFIG. 4, the reaction chamber containing a substrate carrier, accordingto embodiments of the present invention;

FIG. 5B shows a partial cut-away view along X-X of the epitaxial reactorof FIG. 4 with an open gate valve, the reaction chamber containing asubstrate carrier, according to embodiments of the present invention;

FIG. 5C shows an end view of the epitaxial reactor of FIG. 4, where theview is through the open gate valve showing the end of the substratecarrier, according to embodiments of the present invention;

FIG. 6 is a perspective view of a substrate carrier, according toembodiments of the present invention;

FIG. 7 is a simplified representation of a cross section along Y-Y of afirst embodiment of the epitaxial reactor of FIG. 4, according to thepresent invention;

FIG. 8 is a simplified representation of a cross section along Y-Y of asecond embodiment of the epitaxial reactor of FIG. 4, according to thepresent invention;

FIG. 9A is a perspective view of heat lamps and reflector structure ofthe epitaxial reactor and FIG. 9B provides a detailed view of onecorner, according to embodiments of the present invention;

FIG. 10 shows horizontal and vertical cross-sections, along X-X and Y-Ythrough the epitaxial reactor of FIG. 4 and corresponding calculatedtemperature profiles, according to embodiments of the present invention;

FIGS. 11A & B are an illustration of deposition in silicon precursordepletion mode and thickness averaging using cross-flow, according toembodiments of the present invention;

FIGS. 12A, B, C, D, E & F are cross-sections and plan views of a firstembodiment of a substrate carrier and details thereof, according to thepresent invention;

FIGS. 13A, B & C provide detailed perspective views of a secondembodiment of a substrate carrier, according to the present invention;

FIGS. 14A & B provide details of perspective views of a third embodimentof a substrate carrier, according to the present invention;

FIGS. 15A, B & C provide a detailed perspective view and cross-sectionsof a fourth embodiment of a substrate carrier, according to the presentinvention;

FIG. 16 is a perspective view of a gas manifold, according toembodiments of the present invention;

FIG. 17 is a perspective view of a first embodiment of the gas manifoldof FIG. 16 cut in a plane parallel to the plane of the section Y″-Y″,according to the present invention;

FIG. 18A is a perspective view of a second embodiment of the gasmanifold of FIG. 16 cut in a plane parallel to the plane of the sectionY″-Y″, according to the present invention;

FIG. 18B is a perspective view of the gas manifold of FIG. 18A cut in aplane parallel to the plane of the section X″-X″ in FIG. 4, according tothe present invention;

FIG. 19A is a perspective view of a third embodiment of the gas manifoldof FIG. 16 cut in a plane parallel to the plane of the section Y″-Y″,according to the present invention;

FIG. 19B is a cross-sectional view along X″-X″ of the gas manifold ofFIG. 19A, according to the present invention;

FIG. 20 is cross-sectional detail view of the interface between the gasmanifold and the substrate carrier, according to embodiments of thepresent invention; and

FIG. 21 is a cross-sectional representation of a wafer grown on arelease layer over a substrate, according to embodiments of the presentinvention.

DETAILED DESCRIPTION

Embodiments of the present invention will now be described in detailwith reference to the drawings, which are provided as illustrativeexamples of the invention so as to enable those skilled in the art topractice the invention. Notably, the figures and examples below are notmeant to limit the scope of the present invention to a singleembodiment, but other embodiments are possible by way of interchange ofsome or all of the described or illustrated elements. Moreover, wherecertain elements of the present invention can be partially or fullyimplemented using known components, only those portions of such knowncomponents that are necessary for an understanding of the presentinvention will be described, and detailed descriptions of other portionsof such known components will be omitted so as not to obscure theinvention. In the present specification, an embodiment showing asingular component should not be considered limiting; rather, theinvention is intended to encompass other embodiments including aplurality of the same component, and vice-versa, unless explicitlystated otherwise herein. Moreover, applicants do not intend for any termin the specification or claims to be ascribed an uncommon or specialmeaning unless explicitly set forth as such. Further, the presentinvention encompasses present and future known equivalents to the knowncomponents referred to herein by way of illustration.

The present invention may represent a transformational technology forthe production of silicon wafers using epitaxy—the CVD deposition ofsingle crystal silicon from the gas phase provided the cost of theepitaxial deposition is comparable if not lower than the currentconventional processes for silicon wafer production for PV. Usingepitaxial deposition, the three most cost-intensive steps are eliminatedfrom conventional silicon PV technology—polysilicon production, thegrowth of ingots and the machining and wafering of the ingots. Theprocess of the present invention, replacing the three most costintensive steps, addresses the fundamental cost drivers of siliconPV—that of excessive materials usage and complexity of materialproduction, as well as high capital costs at the front end associatedwith polysilicon, ingoting, and wafering. The approach of directlydepositing the silicon provides an alternative to the cost intensiveconventional silicon wafer production technologies while keeping thehigh efficiency associated with monocrystalline silicon PV.

A process flow diagram for the epitaxial deposition of silicon wafers isprovided as FIG. 2. The process includes: providing a silicon substrate(210); forming a release layer on the silicon substrate (220); growingan epitaxial silicon wafer on top of the release layer (230); separatingthe wafer from the silicon substrate (240); and reusing the substrate,after cleaning the substrate to remove any remaining release layer(250). A cross-sectional representation of a wafer 231 grown on arelease layer 221 over a substrate 211 is shown in FIG. 21. Epitaxialtechniques, followed by separation processes, have been described in theliterature as a method to obtain ultra-thin (˜5-50 μm) crystalline solarcells. For example, see Brendel et al. “15.4%-efficient and 25 μm-thincrystalline Si solar cell from layer transfer using porous silicon”Phys. Stat. Sol. (a) 197, No. 2, 497-501 (2003). These methods allow fordramatic reductions in Si usage (by approximately 80% compared toutilizing current commercially available thin Si wafers made by theconventional polysilicon, ingoting and wafering process), whilemaintaining high cell efficiencies with appropriate light trapping andsurface passivation. However, these processes are not cost competitivewith conventional wafer production. The challenge is to provide anepitaxial deposition process at a sufficiently low cost to be able tobenefit from the reduction in silicon usage. The present invention mayprovide such a low cost epitaxial deposition tool and process and adrastic simplification of the supply chain.

Requirements of Epitaxially Deposited Silicon Wafers for PhotovoltaicApplications

In order to provide a technical specification for an epitaxial reactorfor the production of silicon wafers for PV devices it is helpful tounderstand the specific physical requirements of the silicon wafers,which are different from wafers fabricated for semiconductor devicemanufacture.

First consider defect density—this is not a major consideration as longas the defects do not result in reduced minority carrier lifetime orreduce the strength of the wafer. A defect density of 1×10⁵ per cm² isacceptable since it still results in acceptable minority carrierlifetimes (greater than 10 μs), and mechanical strength sufficient toallow processing into cells and modules. In semiconductor applicationsthe defect density requirement is to be less than 1 per cm² mainly dueto the small minimum features which are defined lithographically. Thedefect density requirement limits the growth rate since the defectdensity increases with growth rate and growth rates above approximately1 micron per minute have defect densities above specification. Therelaxation of this requirement for solar applications means that agrowth rate for epitaxial layers of well over 4 μm per minute can beused. This higher growth rate is attainable in embodiments of theepitaxial reactors of the present invention.

Second consider thickness uniformity—in semiconductor epitaxy, therequirement for thickness uniformity across the wafer is less than 2%(driven by stringent depth-of-field restrictions in lithography),whereas for solar approximately +/−10% variation in thickness isacceptable. This relaxed thickness uniformity requirement is attainablein some embodiments of the epitaxial reactors of the present inventionwhich can be operated in silicon precursor gas depletion mode—TCSutilization has been shown to exceed 50% when the reactor is operated indepletion mode.

Furthermore, to reduce manufacturing costs of the silicon wafers, highoutput silicon wafer fabrication systems are needed. Approaches toincrease the throughput of wafers in an epitaxial deposition system haveinvolved very large batches of wafers all being heated and depositedwith silicon at the same time. The main issue with this approach is thatthe volume of such chambers is large and requires long heat-up andcool-down times. Thus to get 200 wafers per hour, for example, the batchsize needs to be over 600 wafers to take into account the heating andcool-down times. This makes the design very complex and so far suchdesigns have not been proven practical. Furthermore, as shown in FIG. 1,these large batch reactors are forced by design to run in the regimewhere there is very little TCS depletion and therefore low TCSutilization. (TCS depletion becomes more and more difficult tocompensate for as the path length of the precursor gases through thewafer carrier increases, such that for very large batch reactorsoperation in a depletion mode is impractical.)

An epitaxial reactor of the present invention takes advantage of themore relaxed defect density and thickness uniformity requirements whencompared with the requirements of epitaxial silicon in the semiconductorindustry. One embodiment of the epitaxial reactor of the presentinvention has the following requirements: (1) a throughput of greaterthan approximately 200 wafers per hour; (2) silicon deposition ratesgreater than 3.5 μm per minute, compared with 1 μm per minute forsemiconductor device applications; (3) thickness uniformity better thanapproximately +/−10% (compared with +/−1% for semiconductor deviceapplications); (4) efficient power utilization—power utilizationcomparable to an equivalent throughput conventional Siemens reactor,which is only roughly 50% of the energy requirement for the conventionalprocess which also includes a large energy budget for crushing, melting,etc of the silicon.; and (5) efficient TCS utilization of roughly 50%(compared with 5% for semiconductor device applications). The keyinnovations that enable the above requirements are described below.

High Throughput Mini-batch Reactor

In embodiments of the present invention throughput is increased using amini-batch concept with multiple mini-batch reactors to a system.Multiple mini-batch reactors have advantages over a single large reactorsince control of substrate temperature and gas flow over the surface ofthe substrates is more readily achieved in a small reactor. Furthermore,multiple mini-batch reactors are more desirable as far as maintenance isconcerned, since it is easier to keep a continuous high throughput whiledealing with both scheduled and unscheduled maintenance. FIG. 3illustrates an example of a silicon wafer epitaxial deposition system300 based on multiple mini-batch reactors 310. Multiple substrates areloaded into substrate carriers at loading station 320 and thentransferred to a pre-heat chamber 330 where the substrates and carrierare heated up to 400° C. The carrier is transported by a transportdevice through tunnel 340 and inserted into one of the mini-reactors310. (Alternatively, the carrier loaded with substrates may be placeddirectly into the reactor at room temperature and heated up todeposition temperatures entirely within the reactor.) The carrier isthen heated to approximately 1,150° C. and the silicon wafers areepitaxially deposited on the substrates. After the silicon depositionprocess is complete, the substrate carrier is cooled to approximately400 to 600° C. and then the carrier is removed from the mini-reactor310, transported through tunnel 340 (for which the sides, top andtransport robot are not shown) and placed in a cool-down chamber 350 forcooling to room temperature before being removed from the system 300. Inthe meantime a carrier with unprocessed substrates is loaded into theepitaxial reactor thus ensuring continuous operation. Removal ofsubstrate carriers from the mini-reactor while the substrate carrier isstill at 400 to 600° C., thus reducing process cycle time through thereactor. The tunnel may be filled with nitrogen gas (less than 1%oxygen) or an over-pressure of nitrogen may be used in the reactor whenthe substrate carriers are inserted and removed, in order to keepundesirable gases out of the reactor. Furthermore, if it is desired tominimize oxidation of the deposited silicon a nitrogen ambient may beused in the tunnel. Note that movement of the hot substrate carrier maybe implemented by a robot with carrier arms with low thermalconductivity, low total heat capacity and low weight; this carrier armboth protects the robot from the heat of the substrate carrier and moreimportantly avoids too rapid a cooling of the substrate carrier.

FIG. 3 is a particular example of a system which has 8 mini-batchreactors. However, these systems may have anywhere between 4 and 10mini-batch reactors—primarily limited by the transport device.Furthermore, other transport devices may accommodate a different and/orwider range of mini-batch reactors per system.

Note that the mini-batch approach allows for use of depletion modedeposition by keeping the path length of precursor gases through thewafer carrier across the surface of the silicon substrates short enoughto be controlled such that the deposition thickness uniformityrequirements can be met. For example, FIGS. 7 and 8 below show a pathlength for precursor gases across the silicon substrates of only twosubstrate widths, for which sufficiently uniform deposition thicknesswith high TCS utilization (>50%) has been experimentally confirmed—seeFIGS. 11A & 11B.

Further to the mini-batch reactor system described above with referenceto FIG. 3, those skilled in the art will appreciate that there are manyvariations of the system that fall within the scope of the presentinvention. For example, the system may be configured without a tunneland include a larger number of reactors aligned to be serviced by arobot with a long linear travel. Furthermore, the reactors may beconfigured for pass-through processing—substrate holders are loadedthrough a gate valve on one side of the reactor and are removed througha gate valve on an opposite side. The latter configuration would requireat least two robots—one for loading and one for unloading. Yetfurthermore, when lower system throughput can be tolerated, the reactorsmay be clustered around a single robot—this has the advantage of asmaller chamber for the robot in which the environment may be readilycontrolled should oxidation of deposited layers need to be minimized.

Epitaxial Reactor

FIG. 4 shows a process chamber 400 for one of the reactors 310. Theprocess chamber 400 comprises a vessel 401 made of stainless steel orother suitable material, a gate valve 402 for inserting and removing asubstrate carrier, lamp modules 403 for providing rapid, spatiallycontrollable heating of the substrate carrier and various apertures 404for providing and removing gases, coolants, etc. As will be clear tothose skilled in the art, many details such as electrical connectors,mechanical adjustment devices, etc. are not shown for the sake of clearillustration of features of the invention.

FIG. 5A is a cut-away along X-X showing the interior of the processchamber 400 with a substrate carrier in processing position. Gasmanifolds 405, heat chokes 505 and gas distribution channels 504 areshown at the top and bottom of the substrate carrier. The substratecarrier is shown in cut-away view such that only one layer of substrates502 are shown on a susceptor 501—multiple layers of substrates arecontained within the substrate carrier as will be apparent from FIGS. 7& 8. FIGS. 5B & 5C show two partially cut-away views of the processchamber 400 with a gate valve 402 through which a substrate carrier 500is inserted and removed.

As indicated above, the substrate carrier is removable from the reactorfor ease of loading and unloading the substrates—through loadlock 402.Further details of substrate carrier removal are provided in U.S. PatentApplication Publication No. US 2010/0263587 to Sivaramakrishnan et al.published on Oct. 21, 2010, incorporated herein by reference in itsentirety.

As discussed below, the temperatures of the reactor components arecontrolled carefully so that silicon deposition occurs only within thesubstrate carrier, and primarily on the substrate surfaces. Furthermore,the substrate carrier may be configured to engage with the gas manifoldin the reaction chamber of the reactor to contain process gases withinthe substrate carrier, thus enabling a low frequency of cleaning for thereaction chamber of the reactor and a high reactor availability.(Keeping process gases away from the windows 721 of the reactor andavoiding silicon deposition on the window surfaces. See FIGS. 7 & 8.)Consequently, only the removable substrate carrier will require frequentcleaning to remove deposited silicon, and this cleaning can readily bedone outside the reactor. Note that the gas distribution channels 504may also have some silicon deposited on their surfaces since the processgases are preheated as they travel through the channels, and thereforethe gas distribution channels may also require cleaning. The gasdistribution channels are incorporated into the removable substratecarrier to allow for ease of cleaning. Furthermore, the heat chokes 505are in a transition region where there may be some small amount ofsilicon deposition and are also included in the removable substratecarrier for ease of cleaning.

The frequency of cleaning required for the substrate carrier depends onthe thickness of silicon being deposited, among other factors, and canreadily be determined by monitoring the process yield—it is expectedthat particulate generation will have the largest yield impact and willconsequently drive the frequency of cleaning. The substrate carrier iscleaned using a standard silicon etch, such as HF/HNO₃.

Epitaxial Reactor: Substrate Carrier

In embodiments of the present invention throughput may be increased by asubstrate carrier that is preferably characterized by low total heatcapacity and high emissivity (black body type)—these properties allowfor the carrier to rapidly be heated and cooled (low heat capacity) andalso for the substrates within the carrier to rapidly reach uniformtemperature (high emissivity). Furthermore, very small substrate carriervolume allows maximum usage of all reactant gases injected into thereactor. This is because (1) most of the substrate carrier volume isconsumed by substrates and susceptor hardware and (2) most of thesurface area inside the substrate carrier that is exposed to processinggases is covered with substrates so that minimal TCS is used to depositsilicon on non-substrate surfaces. Furthermore, the substrate carriermay be configured to reduce heat flow through the surfaces of thecarrier that are not directly heated.

FIG. 6 shows a perspective view of an example of a substrate carrier 500based on these concepts. FIGS. 7 and 8 show cross-sections along Y-Y ofthe reactor 400 of FIG. 4. Note that the cross-sections of FIGS. 7 and 8also show cross sections of the substrate carrier 500 along Y′-Y′ ofFIG. 6. Furthermore, the cut-away view of FIG. 5, also shows parts ofthe substrate carrier. The substrate carrier 500 comprises susceptors501 for holding substrates 502, heat reflectors 503 for reducing heatflow through the ends of the substrate carrier, gas distributionchannels 504 at top and bottom, and heat chokes 505 at top and bottomfor thermally isolating the substrate carrier 500 from the manifolds405. Note that the large areas of the susceptors 501 on the outside ofthe substrate carrier 500 are heated directly by the lamp modules 403through a window 721, and in the embodiment shown in FIG. 7, alsothrough a transparent structural layer 731.

The heat reflectors 503 comprise a series of parallel plates with gas inbetween. See FIGS. 6 and 10—the latter showing the heat reflector incross section. The plates nearest the substrates are generally made ofsilicon carbide or silicon carbide coated graphite and the platesfurthest away from the substrates are generally made of quartz. Duringsubstrate processing the gas between the plates—generally hydrogen—isstagnant. The heat reflectors, so configured, provide high impedance toheat flow and assist in maintaining a stable temperature within thedeposition area of the substrate carrier. Furthermore, when thesubstrate carrier is being cooled, nitrogen gas may be flowed throughthe spaces between plates to decrease the cooling time.

Heat chokes 505 may be made of silicon carbide coated graphite orquartz. The heat chokes may be configured in conjunction with a coolingjacket for efficient removal of heat. The cooling jacket 1090 is shownin cross section in FIG. 10—the cooling jacket is a part of the reactionchamber wall through which coolant flows in order to extract heat.Furthermore, gas flow over the fin-like surface of the heat choke may beused to extract heat during processing, when required, and duringcooling of the substrate carrier.

The silicon deposition region of the epitaxial reactor is discussed inmore detail with reference to FIGS. 7 & 8. Process gases—for which afirst direction of flow is indicated by arrows 701—are directed into theinterior volumes of the substrate carrier, which is enclosed by externalsusceptors 509 and the end caps 506 (see FIGS. 6 & 10). The processgasses flow through the upper gas manifold 405, through the interiorvolumes over the surfaces of substrates 502 and out through the lowergas manifold. The surfaces of the substrates 502 which are exposed tothe interior volumes have release layers, which may be made of poroussilicon as described in more detail below. The epitaxial deposition ofsilicon is on the release layers of the substrates 502. An interiorsusceptor 507/508 separates the interior volumes. The interior surfacesof the susceptors 509, 507/508 are covered with the substrates 502.Thermal radiation 702 is supplied by heat sources, such as lamp modules403. The thermal radiation 702 is conducted through windows 721 to heatthe susceptors 509, 507/508 and the substrates 502 mounted on thesusceptors. The external susceptors 509 are heated and in turn producethermal radiation which heats the interior susceptor 507/508. In orderto cool the substrates and substrate carrier after epitaxial depositionis complete inert gases are flowed through the channels in the substratecarrier. This cooling process also purges any remaining process gasesfrom the substrate carrier prior to removal from the reactor. Nitrogengas at room temperature and atmospheric pressure, or perhaps with aslight over-pressure, is typically used as coolant and purge gas.Furthermore, cooling gases may also be flowed over the exterior surfacesof the substrate carrier within the reactor, and, as discussed above,the cooling gas may also be flowed over the heat chokes and through theheat reflectors to increase the cooling rate of the substrate carrier.

The internal channels through which the process gases flow at standarddeposition flow rates are preferably configured so that the gas flow islaminar and there are no “dead” spaces where particulates mightaccumulate. Furthermore, the width of these channels—spacing betweenopposing substrate surfaces—will be chosen to ensure a suitable laminarflow regime given a particular deposition rate requirement. Thedeposition rate in depletion mode is a function of the flow rate, theTCS/H₂ ratio and the thickness of the boundary layer that exists at thesurface of the wafers. Generally, channel widths are minimized in orderto keep the substrate carrier compact and to keep the boundary layersmall, and this minimum channel width increases as the desireddeposition rate increases in order to maintain laminar flow. Forexample, a channel width of between 8 and 12 mm is suitable for adeposition rate of approximately 4 microns per minute with a TCS flowrate of 10-20 gms per minute. Furthermore, channel widths between 6 and20 mm and flow rates of 2-90 gms per minute may be used, although thedeposition rate will vary considerable over these ranges.

Referring to the arrows 701 in FIGS. 7 & 8 which indicate the directionof flow for the process gases, it should be noted that the flowdirection may be reversed during processing—in which case the flowdirection would be up instead of down as shown in the figures. Whenimplemented during epitaxial deposition this is referred to ascross-flow processing. Cross-flow processing may be implemented invarious ways, for example see U.S. Patent Application Publication No. US2010/0263587 to Sivaramakrishnan et al. published on Oct. 21, 2010,incorporated herein by reference in its entirety. Furthermore,cross-flow processing may also be used during other process steps suchas annealing of the separation layer, and also during cooling.

Furthermore, a structural layer 731 may be added to the outer surfacesof the substrate carrier—attaching to the outer surfaces of the externalsusceptors 509. See FIG. 7. The structural layers 731 provide extrasupport to the substrate carrier and are made of material which readilytransmits the thermal radiation to the external susceptors 509, wherethe radiation is absorbed. The structural layers 731 may be made ofquartz and the external susceptors 509 of silicon carbide, for example.The interior susceptor 507/508 may also be made of silicon carbide.

Yet furthermore, double sided substrates 802—that is substrates that areprepared with release layers on both sides and on which epitaxialsilicon is deposited simultaneously on both sides within the epitaxialreactor—may be placed in a special susceptor plate 508 which permitsexposure of the two sides of the substrates to two different channelsfor simultaneous epitaxial silicon deposition. See FIG. 8. Use ofdouble-sided substrates may allow for an even lighter wafer carrier,hence even faster heat-up and cool-down processes, and thus higherthroughput. The release layers may be porous silicon as described inmore detail below.

The substrate carriers shown in FIGS. 7 & 8 are examples of particularembodiments of the present invention. Further embodiments of substratecarriers according to the present invention may: include more than oneinterior susceptor; and/or be configured to hold substrates for singlesided and/or double-sided epitaxial deposition. Extra interiorsusceptors may be introduced in parallel to the other susceptors,forming extra channels through which process gases flow over thesurfaces of substrates. These extra susceptors are heated by thermalradiation as described above for the case of a substrate carrier with asingle interior susceptor. Furthermore, tilting the substrates—so thatthe substrates are at a small angle to the process gas flowdirection—may be used to affect the thickness of the boundary layer.Specifically, where the channel through which the gas is flowingnarrows, the flow increases and the boundary layer thickness is reduced;thus, tilting the substrate at an angle to the flow direction willincrease the gas velocity and reduce the boundary layer thickness as thegas flows along the tilted substrate toward the end that impinges themost upon the channel. Substrate holders for tilting substrates aredescribed in U.S. Patent Application Publication No. US 2010/0263587 toSivaramakrishnan et al. published on Oct. 21, 2010, incorporated hereinby reference in its entirety.

A substrate carrier such as shown in FIGS. 6-8 and 12 designed to carrytwenty-four 125 mm square silicon substrates may be fabricatedsubstantially from SiC and SiC-coated graphite, and have a total mass ofno more than approximately 42 kilogrammes (without silicon substrates).Furthermore, if double sided substrates are used, this will result in asignificant reduction in weight—perhaps 25-30% for a carrier with oneinterior susceptor, and 50% for a carrier with two interior susceptors.Such a substrate carrier may be heated using dual lamp modules 403 from500 to 1,150° C. within approximately 15 minutes; the cool down time to500° C. is of similar duration. As described above, the depositionsystem may be configured to allow removal of substrate carriers from thereactor while the substrate carrier is still at 400 to 600° C., thusreducing process cycle time through the reactor. Note that siliconcarbide is used for its chemical inertness and purity, and high thermalemissivity. Components which may have silicon deposited on them arepreferably made of silicon carbide since: (1) silicon adheres well; (2)silicon is readily removed from silicon carbide due to excellentselectivity for both wet and dry etch; (3) there is low diffusivity ofcommon impurities in silicon carbide; and (4) good thermal propertiesthat make silicon carbide a good choice for a thermal diffuser.

Epitaxial Reactor: Heat Source

In embodiments of the present invention throughput may be increased atlow cost by a heat source for heating the substrates in the reactorwhich has the following characteristics: (1) low heat capacity to enablerapid heat-up and efficient conversion of electrical energy to heat; (2)spatial control of heat delivered over the surfaces of the externalsusceptors; and (3) efficient reflector to ensure most of the heatgenerated is directed to the substrate carrier. Such a heat source isshown in FIGS. 9A & 9B. The heat source 900 comprises susceptor lamps711, manifold lamps 712, and vertical lamps 902, all housed in areflector 901. The lamps are controlled individually or in groups toenable uniform heating of the substrates and where required to controlthe temperature of different components of the substrate carrier andmanifold. The heat lamps are configured to enable rapid uniform heat-upof the substrates to 1,200° C. and the power to the lamps is adjusted inreal time to enable constant substrate temperature, even while thereactant gases are flowing. An example of a suitable lamp is a tungstenhalogen lamp.

There are two heat sources 900 for each epitaxial reactor, as shown inFIGS. 7 and 8. In one particular embodiment of a reactor each set oflamps is rated at 110 kW, which is suitable for heating 36 siliconsubstrates, 125 mm×125 mm, to 1,150° C., and maintaining thistemperature during processing when gas is flowing through the substratecarrier.

FIG. 10 shows normalized irradiance profiles measured on the outsidesurface of the substrate carrier and the gas manifolds along thedirection of gas flow—plot 1001—and perpendicular to the direction ofgas flow—plot 1002. Note that the irradiance is proportional totemperature, but without accounting for heat losses. The irradiance ismeasured in watts per mm² (measured in the IR spectrum) as a function ofposition across the susceptor, measured in millimeters. Cross-sectionalillustrations of the center part of the epitaxial reactor, showing partof the sections Y-Y and Z-Z, are provided above and below, respectively,the irradiance plots. Note that for convenience of illustration thesecond lamp module (on the opposite side of the substrate carrier) isnot shown in these cross-sections. The plots 1001 and 1002 correspond tothe illustrations above and below, respectively, as indicated by thearrows. The horizontal scale for the illustrations and the abscissa forthe irradiance plots are in correspondence—for example, the side peaksin plot 1001 correspond spatially to the position of gas manifolds 405.

Irradiance plot 1001 shows how the temperature is controlled at the gasmanifolds 405, thermal chokes 505, gas distribution channels 504 and atthe susceptors and substrates. The gas is heated in the manifolds 405 bythe dedicated lamps 712—the gas is heated to safely below thetemperature at which dissociation of precursor gas or deposition fromprecursor gas occurs. The temperature is seen to drop in the thermalchokes 505 which provide some thermal isolation of the substrate carrierfrom the gas manifolds 405. The thermal isolation provided by thethermal chokes allows some degree of independent control of thetemperature of the gas manifolds and the interior of the substratecarrier, as shown in FIG. 10. The temperature is then seen to increasealong the gas distribution channels 504 from the thermal choke to thesusceptors holding the substrates. (In some embodiments the surfaces ofthe channels through which process gas flows are covered withfeatures—such as fins—to enhance heat transfer from the walls of thechannels to the process gas. Such features are preferably engineered toimprove heat transfer to the process gas while maintaining a uniform gasflow across the substrates.) The substrates and susceptors aremaintained at a constant deposition temperature. The temperature of thegas distribution channels, susceptors and substrates is controlled bythe row of lamps 711. Note that the symmetry in the temperature profileis necessary for two reasons: (1) to ensure uniform temperature of thesubstrates, and (2) to permit cross-flow processing, as described above.

Considering gas flow in one direction through the reactor, and withreference to section Y-Y and plot 1001 in FIG. 10, the gas is firstpreheated in one of the gas manifolds and then ideally the gas heats uptowards deposition temperature as it flows along the first set of gasdistribution channels, reaching deposition temperature just before itflows across the substrates. After flowing across the substrates, thedepleted gas then flows along the second set of gas distributionchannels before being exhausted through the second gas manifold. Duringcross-flow processing, the direction of gas flow is reversed and theexhaust manifold becomes the gas supply manifold, etc.

Irradiance plot 1002 shows how the temperature is controlled at the heatreflectors 503, end caps 506, and at the susceptors and substrates. Thesubstrates and susceptors are maintained at a constant depositiontemperature, and the end caps are kept at roughly the same temperature.The temperature is seen to drop through the heat reflectors. Thetemperature of the end caps, susceptors and substrates is controlled bythe rows of lamps 711 in the lamp modules which run the entire length ofthe substrate carrier. The lamps 902 are positioned for heating of theend caps and heat reflectors—the extra heat provided by lamps 902 is toensure that the temperature across the entire width of the substratesand susceptors is the same, and to avoid the temperature dropping at theedges of the substrates and susceptors adjacent the end caps. Note thatthe symmetry in the temperature profile is necessary to ensure uniformtemperature of the substrates.

Note that although the above examples describe situations in which thetemperature across the susceptor is fairly uniform in the region wherethe substrates are held, embodiments in which the temperature ispurposefully varied across the substrates are also envisaged. Thetemperature non-uniformity across the substrates would be used to assistin compensating for TCS depletion. The lamps in the lamp modules may beprogrammed to accommodate such a non-uniform temperature profile,wherein the power supplied to individual lamps or groups of lamps isindependently controlled.

Epitaxial Reactor: Depletion Mode Deposition

In embodiments of the present invention the cost of epitaxial silicondeposition may be reduced by operating the reactor in a TCS depletionmode so as to achieve the high TCS utilization required for low costprocesses. For example, as the TCS precursor gas is flowed across thesurface of substrates the TCS is consumed such that further downstreamas the gas exits the substrate carrier, there is very little TCSpresent. Experiments in a prototype reactor design according to thepresent invention have shown up to 60% TCS utilization. As seen in FIG.1, the deposition rate reduces drastically as the TCS gets depleted.Consequently, the thickness of silicon deposited on the substrates alsoreduces drastically along the direction of gas flow. Thus to compensatefor the reduced thickness the direction of gas flow is reversed so thatthe region which had the low deposition rate now has the high depositionrate and vice versa. See curves 1101 and 1102 in FIG. 11A, which showthe silicon deposition rate on the substrates along the direction ofprocess gas flow—left to right and then right to left, respectively.Curve 1103 shows the average deposition rate for the two depositions,indicating that a relatively uniform film thickness can be achieved,although there is a small peak in the center. Conversely, curve 1104 inFIG. 11B shows a slight dip in the center. FIGS. 11A & 11B show that byvarying the flow rate, a sufficiently uniform film thickness can beachieved; the low flow rate for FIG. 11A is in the region of 60 to 100standard liters per minute (slm), and the high flow rate of FIG. 11B isin the region of 150 to 300 slm. Cross flow may be implemented invarious ways, as described above with reference to FIGS. 7 & 8 and alsosee U.S. Patent Application Publication No. US 2010/0263587 toSivaramakrishnan et al. published on Oct. 21, 2010, incorporated hereinby reference in its entirety.

Although the concept of cross-flow is simple, in practice it isdifficult to use cross-flow to achieve uniform thickness of depositedsilicon on the substrates because the depletion profiles can be highlynon linear—a salient feature of FIG. 1. However, adjustment of theTCS/H₂ ratio, temperature and flow rate so that the thickness profile isroughly linear and then reversing the flow direction, has beensuccessfully implemented to average out the thickness variations. Usingthis technique, a rough averaging out between the flows in oppositedirections achieves the required thickness variation of less than +/−10%while keeping the TCS utilization high.

Epitaxial Reactor: Further Features of Substrate Carrier

Further details of various embodiments of substrate carriers accordingto the present invention are shown in FIGS. 12A-12F, 13A-13C, 14A &14Band 15A-15C.

FIG. 12A shows a cross-section along Y′-Y′ through a substrate carriersuch as shown in FIG. 6. FIGS. 12B-12D show details of the substratecarrier and FIGS. 12E & 12F show plan views of the susceptor in theprocess of substrate loading (FIG. 12E) and with a substrate in place(FIG. 12F). With reference to FIGS. 12A-12C, 12E & 12F the carrier isshown to have outer susceptors 1201 and center susceptors 1203, wherethe susceptors are joined together by connector pieces 1204 as shown.The outer susceptors 1201 carry one substrate each and the centersusceptors 1203 carry two substrates—one on each side; although, thecenter susceptors may also be configured for double-sided deposition(see cross-section of center susceptor 508 in FIG. 8). The substratesare slid into grooves 1208 in the susceptors and into slots 1205 at thebottom of the susceptors and are held in position by securing clamps1202. Heat chokes 505 and gas distribution channels 504 are shown at thetop and bottom of the carrier. An example of the gas flow through thesubstrate carrier is shown by the arrows.

A detail of a securing clamp 1202 is shown in FIG. 12D; the securingclamp has the same cross-section across the entire top edge of thewafer. Furthermore, the slots 1205 at the bottom of the susceptors 1201and 1203, which capture the bottom edge of the wafer, have the samecross-sectional profile as shown in FIG. 12D, except for beinginverted—the shape of the slots serves the same purpose as the shape ofthe clamp, described below, and accommodates reversing the gas flowdirection across the wafer surfaces. Yet furthermore, the grooves 1208which capture the vertical edges of the substrate have the samecross-sectional profile as shown in FIG. 12D, except for having greatertolerance to allow for ease of sliding the substrate in and out—theshape of the grooves 1208 serves the same purpose as the shape of theclamp, described below. The configuration of the securing clamp permitsthicker silicon deposition over the surface of the substrate withoutbridging of epitaxially deposited silicon 1207 between the substrate andthe susceptor. For example, 200 micron wafers may be deposited withoutbridging when the dimensions of the clamp are as follows: A=1.5 mm;B=1.6 mm; and C=0.5 mm. Note that the non-uniform deposition of siliconat the edge of the substrate may be removed using trimming or scribingtechniques prior to separation of the wafer from the substrate—thewafers are deliberately deposited on oversize substrates to allow forthe loss of 1 or 2 mm around the edges.

FIGS. 13A-13C show perspective views of an alternative embodiment of asusceptor plate. Susceptor plate 1301 and removable slide 1302 holdsubstrate 1310 in place. The corners of the substrate 1310 have slots1311 machined as shown in FIG. 13C. The detail of FIG. 13B shows a wedge1303 attached to the removable slide 1302, the wedge fitting into theslot 1311 to hold the wafer in place. Furthermore, there are similarwedges at the other corners of the substrate, as seen in FIG. 13A. Inthe example shown in FIGS. 13A-13C, the susceptor is a center susceptorsuitable for double-sided deposition (see cross-section of centersusceptor 508 in FIG. 8); however, this same approach for holdingsubstrates may also be used for outer susceptors, and for centersusceptors configured for single-sided deposition (see cross-section ofcenter susceptor 507 in FIG. 7).

FIGS. 14A & 14B show a further embodiment of a susceptor plate.Susceptor plate 1401 and removable slide 1402 hold substrate 1410 inplace. The edges of the substrate 1410 have grooves 1411 machined intothem as shown in the detail FIG. 14B. Protrusions 1403 on removableslide 1402 mate with the grooves 1411 to hold the substrate 1410 inplace. Note that protrusions are also located on the bottom inside edgeof the susceptor plate 1401 which mate with grooves 1411 in the bottomedge of the substrate, as indicated in FIG. 14A. In the example shown inFIGS. 14A & 14B, the susceptor is an outer susceptor; however, this sameapproach for holding substrates may also be used for centersusceptors—both for single-sided and double-sided deposition.

FIGS. 15A-15C show details of another embodiment of an outer susceptorplate. The configuration is the same as for the susceptor plate shown inFIGS. 14A-14B, except instead of four protrusions configured to matewith grooves in the substrate edge there are four clamps 1503 forcapturing the substrate 1510—two at the top edge and two at the bottomedge of the substrate. In this embodiment, there is no need for thesubstrate 1510 to have any grooves machined into the substrate edge.FIG. 15A shows susceptor plate 1501 and removable slide 1502 holdingsubstrate 1510 in place. FIG. 15B shows a cross-sectional detail of theclamp 1503, showing how the top edge of the substrate 1510 is captured.The dimensions and angle of surfaces of the clamp 1503 are adjusted topermit thicker silicon deposition over the surface of the substratewithout bridging of silicon between the substrate and thesusceptor/slide at the position of the clamps 1503. Furthermore, thereis a recess 1506 under the edge of the substrate 1510, the recessrunning around the entire substrate perimeter. FIG. 15C is across-sectional view in the Z″-Z″ plane showing the recess 1506, whichis part of the susceptor 1501, under the horizontal edge of thesubstrate 1510. The recess is configured as shown in FIG. 15B along thetop and bottom edges of the substrate, except where the clamps arepositioned. At the position of the clamps the recess is configured asshown in FIG. 15B. The recess is configured to reduce bridging ofsilicon between the substrate and the susceptor/slide during deposition.For example, a recess of approximately 1 mm under the edge of thesubstrate was sufficient to avoid bridging when depositing five 200micron thick wafers on the substrate. (After these five depositions thesusceptor is cleaned to remove silicon build-up.) Note that the recessalso permits some silicon deposition around the edge of the substrate.Furthermore, note that the non-uniform deposition of silicon at the edgeof the substrate may be removed using trimming or scribing techniquesprior to separation of the wafer from the substrate—the wafers aredeliberately deposited on oversize substrates to allow for the loss of 1or 2 mm around the edges.

Furthermore, the recess 1506 around the perimeter of the substrate maybe integrated into the single-sided deposition configuration of thesusceptors/slides of FIGS. 13 and 14.

The susceptor plates shown in FIGS. 12E, 12F, 13A and 14A are configuredfor carrying single substrates, and multiple of these susceptor platesare connected together to form a susceptor plate for carry multiplesubstrates, as shown in FIG. 12A; however, susceptor plates may beconfigured to carry multiple substrates—for example, a susceptor platemay be configured for holding three substrates in a horizontal row, suchthat the susceptor plate 501 could be made up of two such “half” platesconnected one above the other. Furthermore, the susceptor plate 501 maybe a single plate configured to hold all six substrates. See FIG. 5A.

The process of loading substrates follows the following general steps.Substrates are loaded into susceptor plates, the bottom edge of thesubstrate being captured by a slot or other retaining features. Aretaining clamp or slide is then slid into place over the top edge ofthe substrate. The loaded susceptor plates are then assembled in thesubstrate carrier, which may include inserting connector pieces betweenthe separate susceptors. The substrate carrier is then fully assembledby adding top gas distribution channels and thermal chokes. For example,see the cross-sectional view in FIG. 12A.

Epitaxial Reactor: Gas Manifold

A gas manifold is preferred that can act as an injector for the reactantgases but also as an exhaust, so that cross-flow processing can beefficiently implemented. An embodiment of the manifold 405 is shown inperspective in FIG. 16. The manifold 1600 has a body 1601, gas inlettube 1602, gas exhaust tubes 1603, separation plate 1604 for separationof inlet and exhaust gases as explained in more detail below, gasexhaust apertures 1605, gas inlet slit 1606 and alignment ridges 1620for alignment and sealing of the gas manifold to the substrate carrier.

FIG. 17 shows a view of a first embodiment of the manifold of FIG. 16which has been cut in a plane parallel to the plane of the sectionY″-Y″, which is also parallel to the plane of the section Y-Y of FIG. 4.The manifold has a long square section pipe body 1701 through which oneor more apertures 1731—perhaps two or three—have been formed. Theapertures 1731 are connected to external gas supply lines for providingprocess gases to the manifold. A circular cross-section pipe 1715 iswelded to the interior of pipe 1701. Apertures 1731 extend through pipe1715 as well. A gas diffuser plate 1707 with a plurality of evenlyspaced small apertures 1708, formed in two rows along the length of thepipe 1701, separates the inlet part of pipe 1715 from the outlet partand ensures a uniform process gas delivery from the manifold to theinterior of the substrate carrier 500 along the length of the manifoldthrough a multiplicity of apertures 1709 arranged uniformly along thelength of the pipe 1701; although, the arrangement of apertures 1708 maybe varied providing the function of the diffuser plate is preserved.Flow of process gas through the manifold is indicated by arrows 1710 and1711. A plate 1704 is integrated into pipe 1701 in order to ensurecomplete separation of process gases and exhaust gases. Plate 1704includes a large number of apertures 1705 through which the substratecarrier is exhausted; although, in this particular example the apertures1705 are uniformly spaced in rows along the length of the manifold,other arrangements of the apertures 1705 may be used providing efficientexhausting of gas is achieved. Flow of exhaust gases is indicated byarrows 1712 and 1713. The exhaust gases flow along the length of themanifold through chamber 1714 and are removed at either end through anexhaust gas line (1603, for example) attached to a vacuum pump. Notethat the manifold is used in either a process gas supply mode or anexhaust mode only—not in both modes at once. The configuration of themanifold shown in FIG. 17 permits very rapid switch of gas flowdirection without mixing of exhaust and process gases, and also providesa higher impedance gas inlet (suitable for efficient process gaspreheating) and a lower impedance gas exhaust. Furthermore, as describedin more detail below, the substrate carrier may be configured to engagewith the gas manifold in the reaction chamber of the reactor to containprocess gases within the substrate carrier, thus enabling a lowfrequency of cleaning for the reaction chamber of the reactor and a highreactor availability.

Furthermore, the manifold may be configured to increase the path lengthof gas through the manifold in order to improve gas preheating, shouldthis be required. For example, FIGS. 18A and 18B show perspective viewsof a manifold design with a longer process gas path length through themanifold. The manifold has been cut in planes parallel to the planes ofthe sections Y″-Y″ and X″-X″ of FIG. 16; FIG. 18B shows only half thelength of the complete manifold. The manifold has a long square sectionpipe body 1801 through which one or more apertures 1831—perhaps two orthree—have been formed. The apertures 1831 are connected to external gassupply lines for providing process gases to the manifold. A series oftube sections 1816 and 1817 and tube 1818 are attached to the interiorof pipe 1801. Apertures 1831 extend through pipe section 1816 as well.After gas enters the manifold through apertures 1831 it must flow to theend of the manifold to get around the baffle 1817, then back along thelength of the manifold to go through apertures 1819 into pipe 1818. Inpipe 1818 there is a gas diffuser plate 1807 with a plurality of evenlyspaced small apertures 1808, formed in two rows along the length of thepipe 1818, separating the inlet part of pipe 1818 from the outlet part.The diffuser plate ensures a uniform process gas delivery from themanifold to the interior of the substrate carrier 500 along the lengthof the manifold through a multiplicity of apertures 1809. A plate 1804is integrated into pipe 1801 in order to ensure complete separation ofprocess gases and exhaust gases. Plate 1804 includes a large number ofapertures 1805 through which the substrate carrier is exhausted,generally as described above for FIG. 17. The exhaust gases flow alongthe length of the manifold through chamber 1814 to either end of themanifold where they are exhausted through apertures 1832, which areconnected to a vacuum pump by an exhaust line (1603, for example).

FIGS. 19A & B show a variation on the embodiment of the manifold shownin FIGS. 18A & 18B. FIGS. 19A and 19B show perspective views of amanifold design with a longer process gas path length through themanifold. FIG. 19A shows a view of the manifold cut in a plane parallelto the plane of the section Y″-Y″ of FIG. 16; FIG. 19B shows across-section in plane X″-X″ of FIG. 16, and indicates the inlet gasflow through the manifold. The manifold has a long square section pipebody 1901. Gas inlet tubes 1902—perhaps two or three—provide processgases to the manifold. A rectangular section box, with side walls 1916and bottom plate 1921, is attached to the interior of pipe 1901.Contained within the box are a baffle 1917, a plate 1918 with apertures1919, and a gas diffuser plate 1907 with a plurality of evenly spacedsmall apertures 1908, formed in two rows along the length of themanifold. After gas enters the manifold through inlet tube 1902 it mustflow to the ends of the manifold to get around the baffle 1917, thenback along the length of the manifold to go through apertures 1919. Thegas must then pass through a diffuser plate 1907 through a multiplicityof apertures 1908 and then through a multiplicity of apertures 1909 inplate 1921 into the gas inlet slit 1906 which delivers the gas from themanifold to the interior of the substrate carrier 500 along the lengthof the manifold. A plate 1904 is integrated into pipe 1901 in order toensure complete separation of process gases and exhaust gases. Plate1904 includes a large number of apertures 1905 through which thesubstrate carrier is exhausted, generally as described above for FIG.17. The exhaust gases flow along the length of the manifold throughchamber 1914 to either end of the manifold from where the gases areexhausted to a vacuum pump by an exhaust line (1603, for example).

The components of the manifold may be fabricated from quartz or SiC, forexample.

FIG. 20 is a cross-section in a plane parallel to the plane of Y-Yshowing a detail of the engagement of the gas manifold to the substratecarrier. FIG. 20 shows the following parts of the manifold: the bottompart of the manifold body 2001, separation plate 2004 with exhaustapertures 2005, gas inlet slit 2006, exhaust chamber 2014 and alignmentridges 2020. FIG. 20 shows the following parts of the substrate carrier:upper part of heat choke 2010, gas flow divider 2011 (which separatesthe gas flow into the two flow paths through the substrate carrier—seeFIG. 12A) and receiving channels 2012 which are designed to mate withthe alignment ridges 2012. Note that the alignment ridges, and thereceiving channels form a complete seal between the manifold andsubstrate carrier—this is apparent from the view of the alignment ridges1620 in FIG. 16. The trapezoidal section of the alignment ridges andmatching receiving channels enables mating of the manifold and substratecarrier with some tolerance for misalignment as the two parts arebrought together—the sloped edges of the trapezoids guide the two partsinto proper alignment. Note that the manifold and substrate carrier areengaged by moving the parts together in the directions shown by thearrows—either one or both parts may be moved; disengagement involvesmovement of the parts in the opposite directions. Further details ofengagement of the substrate carrier and manifold are provided in U.S.Patent Application Publication No. US 2010/0263587 to Sivaramakrishnanet al. published on Oct. 21, 2010, incorporated herein by reference inits entirety. The trapezoidal section ridges and channels alsoaccommodate the differences in thermal expansion of the materials of themating parts, which may be quartz (manifold) and silicon carbide (heatchoke). The ridges and channels are shown to have very similardimensions in FIG. 20; however, more typically the channels may be 1.5to 2 times wider than the ridges for ease of engagement of the parts,providing this allows for alignment within the alignment tolerance ofthe parts. Furthermore, the ridges and channels are not limited tohaving trapezoidal cross-sections—ridges and channels with othercross-sections can be used providing they have sloped edges. Yetfurthermore, the ridges may be on the substrate carrier and the channelsmay be on the manifold. Furthermore, any number of matching ridges andchannels may be used, as are needed to provide a seal and alignment.

To load a fully assembled substrate carrier into the epitaxial reactorthe following general steps may be followed. The substrate carrier istransported by robot to the epitaxial reactor. One or both of the gasmanifolds is moved up/down to provide room for moving the substratecarrier into place. The substrate carrier is moved into place betweenthe gas manifolds in the reactor. The substrate carrier and gasmanifolds are mated and sealed with the assistance of the alignmentridges and channels as described above—for example, the substratecarrier may be lowered onto the lower gas manifold and then the uppergas manifold may be lowered into position on top of the substratecarrier. The process is reversed for removing the substrate carrier fromthe reactor.

Cost Model Revisited

The reduction in manufacturing cost of silicon wafers that is expectedto be achieved using the present invention will permit costcompetitiveness of single crystal silicon solar cells for theforeseeable future. An embodiment of the silicon epitaxial reactor ofthe present invention is used to demonstrate the commercial advantagesof the invention for silicon solar cell fabrication. A cost model isused which assumes a system with 10 mini-batch reactors, each with acapacity of 36 125×125 millimeters squared substrates. The systemoperates in a TCS depletion mode using heat lamps to control thesubstrate temperature and cross-flow deposition. Growth of 180 micronthick wafers is assumed at a rate of 3.5 microns per minute. The lowthermal mass substrate carrier design allows for heat-up from roomtemperature and cool-down to approximately 500° C. of 25 minutes. (Thesusceptor is removed from the deposition chamber to a cool-down chamberwhen the temperature reaches 500-600° C.)

Using the above assumptions, the time for simultaneous deposition of 180μm thick silicon at a rate of 3.5 μm per minute is 51.4 minutes and theoverhead time for heating the batch of substrates from room temperatureto approximately 1,150° C. and then cooling from deposition temperatureto 500 to 600° C. before removing from the reactor is approximately 25minutes, giving a total time for deposition, including overhead, of51.5+25=76.5 minutesThis gives a throughput for the 10 mini-batch system, with 36substrates, of:36×10×60/76.5=282 wafers per hourThen, assuming an ASP capital depreciation cost for the depositionsystem of $3 million, depreciated over a 7 year period with 90% uptimeand 90% utilization, of3.0×10⁶/(282×0.9×0.9×24×350×7)=$0.22 per wafer

The cost of TCS consumed, assuming $3/kg of TCS and 50% TCS utilizationis $0.20 per wafer. The cost of other gases—mainly hydrogen—is roughly$0.10 per wafer. Furthermore, the cost of other consumables, such aslamps, susceptors and substrates, are estimated at $0.10 per wafer.

The cost of electrical power consumed in heating the substrates assumingthat each of the two lamp units for each of the 10 mini-batch reactorshave an average power rating of 110 kW is given bytotal lamp on time=51 minutes (deposition)+15 minutes (heat-up)=66minutes total energy consumed for 36 wafers=110×2×66/60=242 kWhcost/wafer at 40/kwh=242×0.04/36=$0.26 per wafer

Furthermore, the costs associated with the reusable single crystalsilicon substrate include a cost for growth of the porous siliconseparation layer of approximately $0.05 per wafer, and a siliconsubstrate reuse and reclaim cost, assuming a lifetime of 100depositions, of $0.18 per wafer.

The total cost of the 180 μm thick epitaxial silicon wafer is the sum ofthe costs calculated above$0.22+$0.20+$0.10+$0.10+$0.26+$0.05+$0.18=$1.11 per waferAssuming a solar cell output of 2.65 W (for a 17% efficiency), the costper Wp is approximately $0.40, which meets the target of “total cost ofepitaxial deposition close to $0.70 per wafer” as described at the endof the Background Section.

Many of the assumptions here including the deposition rate and TCSutilization have been verified in a prototype reactor. The above costnumbers were calculated assuming mass production at the modest scale of100 MW per year.

Furthermore, the present invention allows wafers of various thicknessesto be produced, including very thin wafers below 50 μm. Deposition ofthin silicon wafers using the epitaxial reactor of the present inventionhas commercial benefits as described above for 180 μm wafers, and moreso, since the efficiency of conventional wafer production drops due to alarger kerf loss as wafers become thinner.

In addition to the commercial advantages for all wafer thicknesses, anumber of differentiated technical advantages will allow a sustainedcost advantage. These include: (1) perfectly square wafers for highermodule packing density; (2) high quality monocrystalline wafers—nodissolved oxygen as in Czochralski wafers; (3) wafers may be either p orn-type doped; and (4) built-in B-doped BSF (back side field) for highefficiency (higher V_(oc) and lower recombination losses).

Details of Release Layer

Having described the epitaxial reactor and deposition methods of thepresent invention, further aspects of the process steps of silicon waferfabrication according to the present invention are provided withreference again to FIG. 2. In process step (210) a silicon substratewith a (100) crystal plane surface is preferred, since this crystallineorientation is most compatible with formation of a release layer byanodization in a hydrofluoric acid solution. In preferred embodiments ofstep (220), the release layer is formed by anodization in a hydrofluoricacid electrolyte and the anodization is controlled to form a releaselayer with sublayers of different porosities. For example, a highporosity layer is formed below a low porosity layer on the surface ofthe silicon substrate—the high porosity layer providing ease of waferrelease and the low porosity layer providing a good template forepitaxial growth. Further details of the formation of the release layer,including high throughput processing methods for multiple substrates,are provided in U.S. Patent Application Publication No. US 2009/0227063to Ravi et al. published on Sep. 10, 2009, incorporated herein byreference in its entirety. Methods for separation of the silicon waferfrom the substrate rely on the mechanical fragility of the high porositysub-layer of the release layer relative to adjacent layers. Someexamples of suitable methods for step (240) are provided in U.S. PatentApplication Publication No. US 2009/0227063 to Ravi et al. published onSep. 10, 2009, incorporated herein by reference in its entirety. Afterseparation of the wafer from the substrate, the remnants of theseparation layer may readily be removed by chemical and/or mechanicalprocessing, using techniques well known to those skilled in the art.Note that, as described above, the anodization of the substrate surface,epitaxial deposition on top of the anodized surface and separation ofwafers from the substrate may be carried out on both sides of substratessimultaneously, which may provide further improvements in throughput andreduced manufacturing costs.

The substrates are reusable, as shown in step 250—after releasing thedeposited wafer, remnants of the release layer are removed and theprocess flow is followed again, starting at step 210. The number oftimes a substrate may be reused depends on many factors, including theinitial substrate thickness. For example, if a porous silicon releaselayer is used then a 725 micron thick substrate can be reused 50 timesor more before reaching a minimum usable thickness of 400 to 500microns.

Some Characteristics of Epitaxial Silicon Wafers According to thePresent Invention

The silicon epitaxial reactor and deposition methods of the presentinvention have been used to deposit silicon wafers of thicknessesbetween 5 and 250 microns with thickness uniformity measured at ±4 to 5%over a 2×2 array of 125×125 millimeters squared wafers, and it isexpected that wafers of thickness between 1 and 300 microns can befabricated with thickness uniformity better than ±10-15%. These resultsunderline the success of operating in a TCS depletion mode combined withsubstrate temperature control, cross-flow, etc., as described above.Furthermore, it is expected that wafers of thickness up to 500 to 600microns may be made using the present invention. Clearly, wafers belowroughly 50 microns in thickness will require a handle or othersupport—for example see U.S. Patent Application Publication No. US2009/0227063 to Ravi et al. published on Sep. 10, 2009, incorporatedherein by reference in its entirety, for a discussion of handles forepitaxial silicon wafers. Note that if thickness uniformity isprioritized over cost of deposition, then it is expected that thicknessuniformity of ±1 to 2% can be achieved using the epitaxial reactor ofthe present invention.

The silicon epitaxial reactor and deposition methods of the presentinvention have been used to deposit silicon wafers from a TCS precursorwhere the substrate temperature is in the range of 1,000 to 1,250° C.,providing a deposition rate of 3.5 to 10 microns per minute.Furthermore, deposition within the range of 950 to 1,300° C. is expectedto provide a single crystal silicon wafer.

The silicon epitaxial reactor and deposition methods of the presentinvention have been used to deposit silicon wafers with a TCSutilization of 60%, and it is expected that TCS utilization of 70% canbe achieved with further reductions in the non-substrate surface areawithin the wafer carrier. Furthermore, TCS utilization in excess of 5 to10% represents an improvement over the deposition processes used in thesemiconductor industry, and a TCS utilization of 40% or better currentlyrepresents the point at which the process becomes cost competitive inthe solar market. As indicated in FIG. 1, the reactor of the presentinvention can operate with a low Cl/H ratio—even as low as the knee inthe curve of growth rate vs. Cl/H ratio. (As the Cl/H ratio decreases,the percentage of precursor gas consumed to deposit silicon on thesubstrates increases and the process becomes more efficient and lesscostly from the perspective of precursor consumption. Hence, there is adesire to operate the reactor with a low Cl/H ratio in order to reducethe cost of precursor consumed.) The epitaxial reactor of the presentinvention is able to operate with a low TCS to hydrogen ratio since thereactor design allows the deposition conditions to be tailored tocompensate for deposition in the depletion mode where the growth rate ofsilicon may change significantly across a substrate in the direction ofprecursor gas flow.

Further Embodiments of the Present Invention

Although the present invention has been described with respect to toolsand methods for the fabrication of epitaxial silicon wafers from TCSprecursor gas, other precursor gases may be used includingdichlorosilane, silane, silicon tetrachloride, etc.

Although the present invention has been described with respect to toolsand methods for the fabrication of square silicon substrates for solarapplications, the principles and concepts of the present invention areapplicable to the fabrication of rectangular silicon substrates with awide range of dimensions, round silicon substrates (with or withoutflats), etc. In principle, any shape wafer might be formed simply byproviding a template silicon substrate of the desired shape.Alternatively, a large silicon substrate may be used and differentshapes may be defined, using a laser scribing tool for example. Theshapes are then removed using a suitable lift-off technique such asdescribed in U.S. Patent Appl. Publ. No. 2009/0227063 to Ravi et al.published on Sep. 10, 2009, incorporated herein by reference in itsentirety.

Although the present invention has been described with respect to toolsand methods for the fabrication of monocrystalline and multicrystallinesilicon solar cells, the principles and concepts of the presentinvention are applicable to the fabrication of crystalline siliconwafers for a wide variety of uses, including semiconductor devices notrequiring high resolution lithography, certain MEMS (microelectro-mechanical systems) devices such as microfluidic devices, etc.Furthermore, the present invention may be adapted to provide epitaxialsilicon of more uniform thickness—approximately ±1%, at the expense oflower TCS conversion, and will be capable of depositing epitaxialsilicon on the next generation of silicon substrates in thesemiconductor industry—450 mm wafers. However, to achieve this level ofuniformity may require additional variations of the process includingrotating substrates during deposition, or sloping the substrates at asmall angle to the direction of flow, as discussed in U.S. PatentApplication Publication No. US 2010/0263587 to Sivaramakrishnan et al.published on Oct. 21, 2010, incorporated herein by reference in itsentirety.

Although the present invention has been described with respect to toolsand methods for the fabrication of monocrystalline and multicrystallinesilicon solar cells, the principles and concepts of the presentinvention are applicable to epitaxial deposition of a wide range ofcrystalline materials, including GaAs, GaN, Ge, Si—Ge, InGaAs, SiC, etc.Deposition of such a wide range of materials is made possible by theporous silicon separation layer on the surface of the siliconsubstrates, which accommodates a greater lattice mismatch than anunmodified crystalline silicon surface. Alternatively, other substratesmay be used, such as germanium, on which it is expected a suitableporous separation layer can be formed using anodization—the separationlayer must allow deposition of an epitaxial film.

Although the present invention has been particularly described withreference to certain embodiments thereof, it should be readily apparentto those of ordinary skill in the art that changes and modifications inthe form and details may be made without departing from the spirit andscope of the invention.

What is claimed is:
 1. A method of epitaxially depositing silicon waferson silicon substrates in an epitaxial reactor, comprising: providing afirst plurality of silicon substrates loaded in susceptors, saidsusceptors being assembled in a substrate carrier, said substratecarrier being configured to hold said silicon substrates with depositionsurfaces parallel and opposed on either side of a gas flow channel forexposing the opposed surfaces of said substrates to precursor gas;flowing silicon precursor gas over the surfaces of said siliconsubstrates, said flow being parallel to said substrate surfaces, saidflow being linear between first and second gas manifolds at oppositeends of said substrate carrier; and while flowing precursor gas throughsaid channel, heating said substrate carrier to enable precursor gasdissociation on said substrate surfaces; wherein said substrate carrierhas two parallel end caps which define the extent of the flow channel inthe direction perpendicular to the gas flow; wherein said providingcomprises loading said first plurality of silicon substrates into saidsusceptors, said loading including placing bottom edges of saidsubstrates into first slots in said susceptors and placing a second slotover the top edges of said substrates, said top and bottom edges beingaligned in said substrate carrier perpendicular to the gas flowdirection through said gas flow channel, portions of said first andsecond slots being cantilevered over the top and bottom edges of saidsubstrate for shielding; said top and bottom edges from the gas flowthrough said gas flow channel.
 2. The method as in claim 1, wherein saidsilicon substrates comprise a release layer on the surfaces over whichsaid precursor gas flows.
 3. The method as in claim 1, wherein saidsubstrate carrier includes heat reflectors peripheral to said substratesand running parallel to the direction of gas flow through said gas flowchannel.
 4. The method as in claim 1, further comprising separatelyheating the end caps of said substrate carrier.
 5. The method as inclaim 4, wherein said heating is from linear heat lamps positioned oneither side of said substrate carrier and adjacent to and aligned withsaid end caps.
 6. The method of claim 1, further comprising heating saidfirst and second gas manifolds.
 7. The method as in claim 6, whereinsaid heating is from linear heat lamps positioned on either side of saidsubstrate carrier and adjacent to and aligned with said first and secondgas manifolds.
 8. A method of epitaxially depositing silicon wafers onsilicon substrates in an epitaxial reactor, comprising: providing afirstplurality of silicon substrates loaded in susceptors, said susceptorsbeing assembled in a substrate carrier, said substrate carrier beingconfigured to hold said silicon substrates with deposition surfacesparallel and opposed on either side of a gas flow channel for exposingthe opposed surfaces of said substrates to precursor gas; flowingsilicon precursor gas over the surfaces of said silicon substrates in afirst direction from a first gas manifold to a second gas manifold atopposite ends of said substrate carrier, said flow being parallel tosaid substrate surfaces, said flow being linear between said first andsaid second gas manifolds; flowing silicon precursor gas over thesurfaces of said silicon substrates in a second direction from saidsecond as manifold to said first as manifold at o s sosite ends of saidsubstrate carrier, said flow being parallel to said substrate surfaces,said flow being linear between second and first gas manifolds; whileflowing precursor gas through said channel, heating said substratecarrier to enable precursor gas dissociation on said substrate surfaces;wherein said substrate carrier has two parallel end caps which definethe extent of the flow channel in the direction perpendicular to the gasflow; wherein each of said gas manifolds comprises a separation platefor isolating gas inlets from gas exhausts.
 9. The method as in claim 8,wherein said gas inlets of said gas manifolds include a gas diffuserplate for ensuring the uniformity of said precursor gas delivered intosaid gas flow channel.
 10. The method as in claim 8, wherein said gasmanifolds include baffles for increasing the path length of precursorgas through said manifolds.
 11. The method as in claim 1, wherein saidsubstrate carrier includes thermal chokes at the interfaces with saidgas manifolds, said thermal chokes providing some thermal isolationbetween said gas manifolds and said substrate carrier.
 12. A method ofepitaxially depositing silicon wafers on silicon substrates in anepitaxial reactor, comprising: providing a first plurality of siliconsubstrates loaded in susceptors, said susceptors being assembled in asubstrate carrier, said substrate carrier being configured to hold saidsilicon substrates with deposition surfaces parallel and opposed oneither side of a gas flow channel for exposing the opposed surfaces ofsaid substrates to precursor gas; flowing silicon preecursor gas overthe surfaces of said silicon substrates, said flow being parallel tosaid substrate surfaces, said flow being linear between first and secondgas manifolds at opposite ends of said substrate carrier; and whileflowing precursor gas through said channel, heating said substratecarrier to enable precursor gas dissociation on said substrate surfaces;wherein said substrate carrier has two parallel end caps which definethe extent of the flow channel in the direction perpendicular to the gasflow; wherein said substrate carrier includes an interior susceptorconfigured to hold a second plurality of silicon substrates, saidinterior susceptor dividing said gas flow channel into two parallel gasflow channels, the surfaces of said second plurality of substrates beingparallel to the surfaces of said first plurality of substrates; whereineach of said second plurality of silicon substrates have a firstdeposition surface exposed to a first of said parallel flow channels anda second deposition surface exposed to a second of said parallel flowchannels, said first deposition surface and said second depositionsurface being mutually parallel and parallel to said flow of precursorgas through said parallel flow channels.
 13. The method as in claim 12,wherein said substrate carrier includes first and second gasdistribution channels between said first and second gas manifolds andsaid susceptors for splitting said gas flow into said two gas flowchannels.
 14. The method as in claim 13, further comprising heating saidfirst and second gas distribution channels.
 15. The method as in claim14, wherein said heating is from linear heat lamps positioned on eitherside of said substrate carrier and adjacent to and aligned with saidfirst and second gas distribution channels.
 16. The method as in claim14, wherein said first and second gas distribution channels includechannels decorated with features for enhancing heat transfer to said gasflowing through said channels.
 17. The method as in claim 8, furthercomprising sealing said first and second manifolds to said substratecarrier, wherein said first and second manifolds have continuous sealingridges, said substrate carrier having corresponding continuous receivingchannels and wherein said sealing ridges and receiving channels havesloped sides for providing alignment tolerance between said manifoldsand said substrate carrier as said manifolds and substrate carrier arebrought together.
 18. The method as in claim 1, wherein said susceptorsare configured with recesses under the edges of said substrates forreducing bridging of deposited silicon between said substrates and saidsusceptors.
 19. The method as in claim 12, wherein each of said secondplurality of silicon substrates comprises a release layer on said firstdeposition surface and a release layer on said second depositionsurface.